Field emission devices ("FEDs") or micro-vacuum tubes have gained recent popularity as alternatives to conventional semiconductor silicon devices. Typical advantages associated with FEDs are much faster switching, temperature and radiation insensitivity, and easy construction. Applications range from discrete active devices to high density static random access memories, displays, radiation hardened military applications and temperature insensitive space technologies, etc.
Historically, the literature on field emission devices principally focused on process problems associated with producing the sharpest vertical emitter tip (e.g., with photolithography), and controlling cathode to anode and cathode to gate distances by achieving self-alignment between these elements.
Recently, lateral field emission devices have emerged as an alternative to traditional vertical emitter devices. In U.S. Pat. No. 5,233,263 entitled "Lateral Field Emission Devices," issued Aug. 3, 1993, and U.S. Pat. No. 5,308,439 entitled "Lateral Field Emission Devices And Methods Of Fabrication," issued May 3, 1994, lateral field emission devices employing a horizontal thin-film emitter are described. The sharp radius of curvature around the edge of the thin-film emitter produces the high intensity electric field necessary to cause the emission of electrons. In specific regard to the details of the devices described, the emitter tip is always separated from an anode by a distance of approximately 1 micron. In one embodiment, a light emitting FED is created by replacing the anode with a conductive-type phosphor. Electrons are thus transferred into the phosphor causing an emission of light.
These devices have several limitations when used as display elements. The large distance between the emitter tip and the anode results in a large voltage potential being required to excite emission of electrons from the emitter tip towards the anode. Due to the high voltage potential, careful control of the environment between the emitter and the anode is needed so as to avoid degradation of the emitter. For example, the device may be disposed in an evacuated atmosphere or in an inert gas. In regard to further device limitations, when the anode is replaced with a phosphor, ballistic steering effects due to electric fields deflect emitted electrons downward towards a metal extraction anode disposed below the phosphor. Due to the inherent resistance of conductive-type phosphors, coupled with the relatively large volume of phosphor electrons must travel through to reach the extraction anode, even higher voltage potentials are required, which hinders extraction of electrons from the phosphor.
In U.S. Pat. No. 5,144,191 entitled "Horizontal Microelectronic Field Emission Devices," issued Sep. 1, 1992, another lateral field emission device is described. Again, the distance between the emitter tip and anode is on the order of 1 micron, thereby having the aforementioned problems associated therewith (i.e., large operating voltage, emitter degradation, and requirement of a controlled ambient environment). In one embodiment, the anode is replaced with a conductive-type phosphor for creating a light emitting field emission device. This embodiment suffers from further problems. The phosphor anode (i.e., composed entirely of a phosphor) is electrically resistive, making it less efficient in attracting electrons theretowards. Furthermore, the increased resistivity of the phosphor anode hinders the efficient extraction of electrons therefrom. Taken together, these problems decrease the efficiency of the device, and increase the voltages necessary for operation.
In summary, high operating voltages limit the usefulness of FEDs in low voltage applications such as portable computers. Moreover, a requirement that the FED be disposed in a vacuum (or other inert gas environment) adds to the complexity and fabrication costs of the device. The structure and methods of fabrication of the present invention contain solutions to the aforementioned problems.